This invention relates to apparatus which receives a signal of unknown frequency and provides an output signal the value of which represents the frequency, and in particular it relates to such frequency determining apparatus which is suitable for use over a wide range of frequencies.
One form of frequency determining apparatus is the frequency to digital converter which provides a digital output signal representative of the frequency of the input signal. Frequency to digital converters are well known. One type of frequency to digital converter operates by determining the period of the input signal from two succeeding pulses or cycles and using this time period to count pulses from a stable, high frequency pulse generator. The count of pulses is inversely proportional to the frequency and hence the count can be used to provide a digital representation of the input signal.
It is desirable in such frequency to digital converters to have an output available at reasonably regular intervals regardless of whether the input frequency is high or is low. This is of particular significance when the output is to be used as part of a closed loop feedback system, for example, a speed control system. However, prior art frequency to digital converters of this type provide output information at widely different intervals depending on the frequency of the input signal. In addition, a finite time is necessary, after receiving the input signal, to provide the digital representation thereof. As the frequency of the input signal increases and the period becomes shorter, there may be insufficient time to complete the calculation of the digital representation of frequency.